The rapid advance of information technology has created the need for high performance information displays having high spatial resolution, wide viewing angles, rapid response, low weight, low volume, low cost and low power usage. Conventional cathode-ray tube (CRT) displays satisfy the performance requirements but do not generally meet desired weight, volume, and power usage targets. Flat panel displays using liquid crystal (LC) technology represent an alternative to conventional CRT displays. However, due to the limitations of current LC technology, existing flat panel displays do not provide at this time the performance level characteristic of CRT displays.
Current flat panel display technology mainly utilizes nematic liquid crystal materials driven through an active or passive matrix. These displays offer restricted viewing angles, smearing of fast-moving images, restricted overall dimensions, and always consume external electric power to maintain the image. As an alternative, displays using ferroelectric liquid crystal materials overcome many of the limitations of the nematic liquid crystal displays. Surface stabilized ferroelectric liquid crystal displays offer hemispherical viewing angles and a rapid, bistable response. These characteristics potentially make ferroelectric liquid crystal displays superior to nematic liquid crystal displays.
The advantages of ferroelectric liquid crystal displays derive in part from the properties of the ferroelectric liquid crystal material. In its smectic C* state, a ferroelectric liquid crystal material exists in a layered molecular orientation with a molecular director vector n tilted from the layer normal by temperature-dependent angle .theta.. Dipolar contributions of individual molecules within a layer result in a spontaneous polarization vector P.sub.s linearly coupled with the tilt angle .theta.. From layer to layer, the molecular director continuously rotates at a constant tilt angle .theta. around the layer normal due to the chirality of the smectic C* phase. One complete rotation of the director occurs through a distance known as the pitch.
If the ferroelectric liquid crystal material is contained within a cell of thickness comparable or less than the pitch of the material, the material assumes a molecular orientation wherein the molecules are parallel to the cell walls but exist in two distinct domains. In each of these two domains, the molecular director has a distinct orientation. The material contained in this way is referred to in the art as a surface-stabilized ferroelectric liquid crystal. The relative surface area covered by the domains vary in response to an externally applied electric field due to the linear interactions between the field and the spontaneous polarization of the material. When viewed through crossed polarizers the domains reveal electrooptical switching in response to changes in the electrical field. Advantageous features of this switching are bistability due to the two possible orientations of the molecular director, fast switching time due to strong linear coupling between the electric field and the spontaneous polarization, and a hemispherical viewing angle because in both possible director orientations the molecules form uniaxial birefringent plates that can be easily compensated.
A drawback of ferroelectric liquid crystal displays is their tendency to exhibit so-called "zig-zag" defects. In fabricating the displays, the zig-zag defects generally appear during cooling of the liquid crystal materials to reach the ferroelectric smectic C* phase or as a result of mechanical deformation of the liquid crystal display. An understanding of the source and nature of these defects is gained by considering the makeup of a typical liquid crystal light modulating cell. As is well-known in the art, a typical cell consists of two opposed glass or plastic substrates each having an electrode on its inner surface. A polymer alignment layer is disposed on each of the electrodes. When rubbed in a certain manner the polymer layer induces alignment of the molecular director in the liquid crystal material contained within the cell. Along with the alignment inducement, the polymer layer also induces an angular displacement of the liquid crystal molecules with respect to the polymer layer. The induced angle, or pre-tilt, is characteristic of a given polymer and is typically 2-3 degrees for conventional alignment polymers. When both alignment layers are rubbed in the same direction, the interaction of the polymer alignment layer with the ferroelectric liquid crystal in its smectic C* phase typically results in a chevron-like smectic layers orientation. Usually, two chevron patterns, each oriented in an opposite direction, are generated between the substrates. The boundary between macroscopic areas having different chevron patterns appears as a zig-zag defect using polarized transmitted light microscopy. If the layers are rubbed in opposite directions the resulting molecular orientation shows an increased number of zig-zag defects. These zig-zag defects are undesirable as they reduce the image contrast in ferroelectric liquid crystal cells, thus limiting the use of ferroelectric cells in various applications. It is therefore desirable to provide a ferroelectric liquid crystal cell wherein the liquid crystal in the smectic C* phase exists only in one chevron orientation.
Various approaches exist to overcome the zig-zag defects include treatment in a high frequency electric field, shearing of the cell substrates and gradient cooling. These approaches are not, however, compatible with existing liquid crystal display fabrication technology. Hanyu, U.S. Pat. No. 5,189,536, teaches a method to prevent zig-zag defects that is compatible with existing fabrication technology. This method relies on increased pre-tilt angles up to 10-15 degrees between the ferroelectric liquid crystal material and the alignment layer. Unfortunately, this method results in displays having increased switching times required for bistable response.
Therefore, a need exists for a method to prevent the appearance of zig-zag defects in ferroelectric liquid crystal displays. Moreover, there is a need for such a method that is compatible with existing display fabrication technology without compromising the performance of the displays.